Many physiologically active substances such as pharmaceuticals, agricultural chemicals, perfumes, and sweeteners are alcohols having an asymmetric carbon atom or compounds having a partial structure of such an alcohol (hereinafter referred to simply as “alcohols”). There may be optical isomers in such a compound. However, there may be a significant difference in the degree of physiological activity among these optical isomers. Some isomers exhibit physiological activity quite different from others. Therefore, development of a method for separating an optical isomer mixture of alcohols easily without failure has been desired.
As an example of optically resolving an alcohol, Synlett., (6), 862 (2000), J. Org. Chem., 64, 2638 (1999), and the like describe a method comprising allowing one of optical isomers to remain as the alcohol and transforming the other optical isomer into an ester derivative in a natural optically active environment (for example, internal organs of animals containing an esterified enzyme or hydrolyzed enzyme). However, since such an enzyme does not have chemical stability, in particular, thermal stability, the enzyme cannot be used under high temperature conditions. Further, it is difficult for the enzyme to be generally and widely accepted due to its high cost and difficulty in being procured in a large amount.
Tetrahedron., Lett., 35, 4397 (1994) reported an experiment in which an ester prepared by condensing a carboxylic acid having an asymmetric carbon atom with an alcohol was separated into individual diastereomers by silica gel column chromatography. In principle, this is optical resolution of an alcohol.
However, since there are no general rules or principles for producing a highly separable diastereomer mixture, the method cannot be generally applied. And the mixture can rarely be separated into two optical isomers without being influenced by an external optically active factor, such as in the case of spontaneous resolution. General rules for separation do not exist. Accordingly, in almost all cases, it is highly difficult to speculate whether or not an optical isomer mixture of alcohol and the like can be separated into optically active compounds. The mixture is not easily separated in almost all cases.
The present invention has been achieved in view of this situation and has an objective of providing a novel 2-oxabicyclo[3.3.0]octane compound which can be used as an optical resolving agent of an optical isomer mixture such as alcohol, a process for producing such a compound, an optical resolving agent containing at least one 2-oxabicyclo[3.3.0]octane compound, a method of separating a diastereomer mixture, and a method of optically resolving alcohol using the optical resolving agent.